The primary purpose of a container is to contain. When a container fails to do so, the result is unpleasant for the consumer. This is particularly true when the container contains a liquid. In that case, any leak in the container may cause spillage. Conversely, any leak enables microorganisms to enter.
In the case of carbonated beverages, any leak will tend to cause the beverage to go flat. If the leak is a slow one, the consumer may not even know that there was a leak. In that case, a consumer may be prompted to avoid future purchase of a particular brand under the false assumption that drinks sold under that brand are insufficiently carbonated, even though the consumer's experience may have been based on anomalous product sample.
Of particular concern are leaks in large volume containers such as beer kegs. A beer keg is often a centerpiece at a party. Flat beer at such a party will tend to not only spoil the party atmosphere but impart a negative impression to many people under the most unfavorable circumstances.
Accordingly, once a container has been filled, it is useful to confirm that the container does not have any sort of slow leak before releasing it to market.